Very interesting thread! I have been doing extensive research on the border region of what is called Raja-Karjala for the last five years and there are many stories about the border and its impact on the people in this area, which was mainly the north-western border of Suojärvi county, the eastern border of Suojärvi county which was called Hyrsylä-mutka, and also the eastern border of Salmi county.
Quite incredibly before 1920 timeframe, there was an large amount of movement between the Russian and Finnish (which before 1918 was a Russian autonomous region called a Grand Duchy) sides. In fact in some villages the roads into Russia were actually better than any roads that lead west into Finland, and the communities relied more on the Russian side for trade, and religious associations. The border prior to 1918 was in fact invisible in many ways except for taxation reasons.
The border was still quite porous until the late 1920's - early 1930's, and marriages between women and men on each side continued until the mid-1920's. A women on the Russian side would come to her husband in Finland, and Finnish women would go to here husband on the Russian side. The sentence implies differences but in fact the people were all from the same tribe, the Karelians, they just paid taxes to different state governments basically. They spoke the same language, had the same customs, etc., etc. and quite often had extended family connections.
There are many stories about how the border split families in the 1930's, when things got more tight and passing back and forth became much harder, though refugees (or pakolainen) still arrived on the Finnish side from what was now Soviet Russia, and the occasional Finnish person went east to Soviet Russia to follow the dream of the utopian society that was talked about in the propaganda of the Soviets at that time. Life in Raja-Karjala was a tough life, and very primitive in many ways, though things started to improve by the late 1930's just prior to the war, and calls to the utopia of Soviet Russia enticed some people. But despite the tough life there was also a strong sense of community and loyalty in the area, and that ran deeper than the politics of either country.
Someone mentioned above 20 people caught in this area at the start of the Winter War, the number is more over 1,800, with ultimately around 1,700 returning to Finland in May 1940. Originally from November 30th, 1939 to early February the civilian population was allowed to remain in their homes and villages, but in early February 1,768 of these civilian prisoners were taken to two "transit" camps on the Russian side of the border, where they remained until May 25th, 1940. While many civilians died at the camps, and in the two months before, there were also 4 civilians there were taken away at the camps by the Soviets and that were never seen again.
There are many stories about the border, especially smuggling, night time visitors that crossed, and also espionage from both sides. Several residents of Finland were abducted by the Russians on the Finnish side, most in broad daylight, some of these men died in prison camps being accused of crimes against the Soviet state. A wonderful women in her 90's today talked about when being younger, at night if you heard rustling in the bushes you just ignored it knowing it was someone from the Russia side over to see family. Some of the few poignant stories include ones from 1935, when the Soviets liquidated many of the small villages that were very close to the Finnish border, the people being sent to Siberia. The Soviets feeling that these very close villages were a threat to their security as most people in these villages were related, quite closely, to the corresponding villages on the Finnish side. In the village of Posuada on the Russian side, during the liquidation a mother gave her child to her sister on the Finnish side, passing her over the border late at night before they were forced to leave, feeling that she would have a better life in Finland. The reports of that evening say the villagers of Hyrsylä on the Finnish side, heard their cousins on the Russia side singing laments as there were forcibly taken away to Siberia. Another story relates to a women who was caught with the civilians at the start of the Winter War. She lived in the village of Hautavaara and in February was taken away to the "transit" camps, this voyage took some hours in bitter cold weather and some died on the journey but during the start of the journey the convoy stopped at the village of Veskelys on the Russian side, where the prisoners were given some weak soup, and this women then saw her mother and sisters who lived in Veskelys, as the mother had been told that the prisoners were coming through, the mother could only see her daughter briefly but could do nothing to help her and her grandchildren that were with her daughter from Hautavaara. This was the last time this women from Hautavaara ever saw her mother, though she and her children returned to Finland in May 1940.
There are many more stories like this. What I have found very interesting in my research, was that even though after 1920 the border was closed, and truly effectively closed by the mid-1930's, familes, which were mothers and daughters, sisters and brothers, even fathers and sons, somehow were able to keep up on news with each other, even through any affects of Soviet rules, food was sent to the Russian side, and letters smuggled back and forth.
One reasons I believe this was possible is that until the mid-1930's and even after that to a level, the guards and security on the border were in fact also Karelian men, who spoke the language but more importantly were "cousins" of the families on each side. There were of course hard-core believers in the Soviet system, who were Karelian, and they did everything to put down and sabotage the Finnish side, but most were just young men conscripted to serve and who really didn't have a strong attachment to the greater Soviet dream but were Karelians. And of course there was much bribing, and given the harder conditions on the Soviet side you could see how the right items could get you what you needed with regards to news, and passing of letters and information. It is written that the amount of booze that flowed from the Russian side to the Finnish side in turn for flour, and other basic commodities was that it was a river flowing across the border!
The war experience is similar, especially at the start, when you had many Karelians on the Russian side, having to fight their cousins on the Finnish side. Russia had no time for sentimentality, but it also needed Karelian speakers to help. There are stories of Russian soldiers from Karelia, especially those interrogated on the Finnish side, that talked about not understanding the war and the difficulty in fighting their own; there is also much about Russian POWs, especially from Karelia that tried to stay in Finland after the war, many did, escaping the transport trains back to Russia. All POW's were to be returned as part of the peace agreement.
I would also add some information about the time just prior to the war and evacuation of the civilians. This is a rather sensitive subject as I think the truth has never really been figured out. It is known that in the last weeks of November, the civilians on the Finnish side in these border areas were told at one point to pack up and prepare to evacuate west, and most did, and many traveled by train to Suvilahti, where the main train station west was, and other areas but were then turned back and sent home, being told not to worry the war was not going to come. Of course some made their own decisions and went to family or other areas in the west but many did not have that option, not having family, not having money and also not knowing what to do with the livestock, etc. So this lead the tragedy of November 30th, when the Soviets attacked, and these border areas, with very little defense were instantly captured by the Soviets and the civilian population caught behind enemy lines. Some have said this was an intention of the Finnish government, to not give any signal to the Soviets that the Finns were prepared for the attack, though this has not been proven one way or the other. What ever the truth, these civilians suffered much and many died, and ultimately it could have been at a minimum partially avoided if a proper evacuation had taken place.
I guess I will end for now...sorry for the long post!